High Cycle Fatigue Life Assessment of a Heavy Duty Diesel Engine Cylinder Head

Among all critical components in heavy duty diesel engines, cylinder head is generally counted as the most challenging part due to the complex loading conditions. This component experiences both thermal and mechanical stresses. Temperature difference between combustion gases and cooling water induces significant thermal stress in the flameface, while the firing pressure builds up cyclic mechanical stresses superimposed on the former thermal stress. On the other hands, cylinder heads are geometrically complex and generally contain many small fillets and blends. These small geometrical features and complex loading condition cause multiaxial stress state which should be considered during High Cycle Fatigue (HCF) life assessment. In this study, a detailed finite element analysis has been conducted on a heavy duty diesel engine cylinder head, which is followed by HCF life assessment. In order to predict the HCF safety factor, a post-processing routine has been developed using ANSYS Parametric Design Language (APDL) based on a normal stress critical plane approach. This method is shown as a promising approach for multiaxial HCF life assessment of a wide range of engineering metals. Moreover, normal stress critical plane approach is known as a computationally time efficient approach for the huge finite element models. Notch effect has been considered by introducing the relative stress gradient parameter, which is obtained from the best fit of fatigue test data for different notch geometries. Finally, the effect of HCF on Low Cycle Fatigue (LCF) crack growth has been discussed.